Microbial survivability during repeated extreme dry-wet cycles determines CO2 emissions after rewetting of dried soils in humid temperate forests

Shifts in precipitation patterns with less frequent rain events accompanied by global warming will trigger soil drying and rewetting, even in humid regions. Because rewetting of dried soil provokes pulse carbon dioxide (CO2) emissions from soils, the chronic soil dry-wet cycle (DWC) in humid regions may provide positive feedback, contributing to global warming. In this study, we aimed to reveal the effects of repeated DWCs on soil CO2 emissions, and the factors affecting emissions after rewetting in humid temperate forests. Experimentation included incubation of soils under five sequential DWCs. CO2 emissions from the soils were measured throughout the incubation period, except during periods of drying. Soil extractable organic carbon (EOC) and microbial biomass carbon (MBC) were also measured at three hours and at five days after the first, third, and fifth rewetting. Rewetting of dried soil significantly increased CO2 emissions during the first DWC, whereas the size of the pulse CO2 emissions after rewetting decreased with an increasing number of subsequent cycles. Soil rewetting decreased soil MBC and increased EOC, and the EOC concentration decreased during each subsequent wet period. Based on path analysis, MBC three hours after rewetting was strongly and positively correlated with CO2 emissions in the following five days in the dry-wet treatment (regression coefficient β = 0.710, p < 0.001). The results suggest that microbial survivability to soil DWCs, rather than sudden labile carbon supply, determines the response of pulse CO2 emissions from soils after rewetting during repeated soil DWCs in humid regions.